1. Field of the Invention
This invention relates generally to an improved process for etching polysilicon and, more particularly, to a process of pre-mixing CFCl.sub.3 (Freon 11) and SF.sub.6 for etching polysilicon.
2. Background Art
The manufacturing of semiconductor chips and related thin film circuitry involves the etching of different layers such as polysilicon and silicon. Typically, the area to be etched is masked by material such as photoresist with the mask forming a pattern of lines and areas exposing the layer to be etched. Earlier methods accomplished this etching by a wet chemical process using large amounts of various acids, alkalis, organic solvents and the like which contacted the exposed surface. However, many times this wet process resulted in contamination by impurities contained in the chemicals and swelling of the resist film used as a mask causing an irregular shape and occurrences of undercut. Undercutting occurs when the etchant acts horizontally as well as vertically, thereby removing material below the photoresist.
As technology advances and the trend toward miniaturization continues, an increased degree of integration requires more precise etching processes. More recent processes employ gas plasmas, particularly fluorine based gases selected from the saturated halocarbon series such as CF.sub.4, CCl.sub.4, and BCl.sub.3. These gases are mixed with inert gases such as O.sub.2, N.sub.2, or Ar in a chamber in which the etching is to be accomplished.
One particular known process comprises the mixing of CFCl.sub.3 (Freon 11) and SF.sub.6 in the etching chamber which results in etched geometries with less than 0.3 micron total resist mask undercut. Freon 11 is stored in a tank in liquid form and remains a liquid at ambient temperatures, approximately 20.degree. C., because its boiling point is 23.7.degree. C. at one atmosphere pressure. As Freon 11 boils off creating a vapor, the vapor is removed along a line to a mass flow controller (MFC). The MFC precisely measures and provides for a predetermined flow of Freon 11 vapor to the etching chamber. The SF.sub.6 is stored in a second tank in gas form and is also transported along a line to another MFC, wherein a predetermined flow is provided to the etching chamber. However, a temperature or pressure gradient along the line transporting the Freon 11 vapor may cause the vapor to condense into a liquid. This condensed liquid may enter the MFC, causing temporary or permanent failure due to liquid ingestion. Furthermore, it is possible for the liquid to accumulate causing a blockage in the line, thereby preventing any Freon 11 vapor from reaching the MFC. In either case, the end result is production yield loss, costly system downtime, and costly MFC replacement if damage is permanent. Attempts to eliminate the occurrence of this condensation by controlling the temperature of the line have not always been successful. Furthermore, the accuracy of the mix is dependent upon the correct operation of the two MFCs.
Thus, what is needed is an improved process for mixing Freon 11 and SF.sub.6 that prevents failure of the process due to liquid forming in the vapor transportation lines, and where the accuracy of the percent mix of gases is independent of the MFCs.